This invention relates to a cyclic drilling machine in which one drilling process is carried out, instead of being performed in a single stroke, by gradually deepening a bore-depth by means of repeating reciprocative movement of a tool for a plurality of times before completing the drilling process.
More particularly the invention relates to a cyclic drilling machine, wherein when a tool which has been, after having been advanced forward to a little short position to a workpiece at rapid speed and then switched to slow drilling speed for a drilling operation, once returned at rapid speed and advanced forward again it is moved as far as a little short of the terminal of the previous drilling cycle at rapid speed before being switched down there to the drilling speed so that the tool can be prevented from any damage.
A drilling machine of this category disclosed in a Japanese publication TOKU-KO-SHO No. 52(1977)-26834 is provided in general with feed means for reciprocating a quill which retains a spindle, a guide rod disposed in parallel with the quill for being moved therewith, a position memory member retained on the guide rod for memorizing a position where previous drilling operation was interrupted, by the action of a spring which is slidable against friction force along the guide rod, a feed control switch disposed on a frame for switching the rapid feed speed of the quill, on the way of advancing, down to the slow feed speed upon engagement with the position memory member, and an engaging portion disposed on the frame for blocking the movement of the position memory member when the latter engages on the former in the course of advancing of the quill. When the position memory member engages the feed control switch while the tool is advanced forward at rapid feed speed, the feed speed of the tool is switched down to the slow feed speed and the position memory member is further advanced by a small distance until the position memory member is halted owing to engagement with the engaging portion. The position memory member is therefore relatively moved to the guide rod. When the quill is returned from a midway of drilling operation, the position memory member is returned together with the guide rod. When the quill is advanced again the position memory member is engaged with the feed control switch, so that the feed speed of the quill is switched down from the rapid feed speed to the slow feed speed at a position this side of the previous drilling operation interrupted position by the movement amount of the position memory member after engagement thereof with the feed control switch till engagement thereof with the engaging portion.
As the position memory member is retained on the guide rod due to the action of a spring, in the conventional machine as mentioned above, possible loosening of the spring because of usage thereof for a long time, may cause an unexpected happening of accidental moving of the position memory member owing to its dead weight, if and when the whole of the apparatus is placed vertically or under a certain inclination, or uncertain operation of the feed control switch by the position memory member. This will cause an uncertain speed switching of the tool from rapid to slow. In such an instance the tool is liable to strike against or come to impact the workpiece at the rapid speed, being thereby seriously damaged, to the great disadvantage. Besides, the feed of the tool is performed by the fluid pressure actuator, while the reciprocation thereof is controlled according to electric signal from the feed control switch or the like, in the abovementioned prior art, and the switching from the slow feed to the rapid feed is made by a control valve. Such structure of the conventional machine necessitates the control valve, electromagnetic means for operating the same, pipings as passages for the fluid pressure, etc., which inevitably enlarges the size of the apparatuses.